Molecular Dynamics Simulation Study On The Interfacial Properties Of Polymer Nanocomposites

Polymer nanocomposites(PNCs),typically containing one or more nanoparticle(NP)components within a polymer matrix,are known for superior mechanical,electrical,and optical properties especially relative to traditional composites with micron scale additives.The NP size is expected to play a central role in this context since the material properties are driven by the NP surface-to-volume ratios.The present paper focuses on large NPs of typical size of hundred nanometers.In past two decades,a class of ultra-small NPs have received considerable attention.In such systems,the material properties are largely determined by the structural and dynamics properties of polymer-NP interfaces.However,the unusual viscoelastic behavior of these PNC with small NPs results from a number of interconnecting factors,such as NP size,shape,dispersion state,and,more importantly,topological interactions between polymer chains in the bulk and NP(or the chains interfacing NP).Due to the lack of advanced experimental characterization,the study of crucial interfacial properties of PNC remains an extremely challenging task.In this thesis,by using molecular dynamics(MD)simulations,we have investigated the interfacial structures and dynamics of three curated PNC systems with small NPs.The size of OAPS(octaaminophenyl polyhedral oligomeric silsesquioxane)nanoparticle is typically?2 nm,which shows different structural and viscoelastic properties compared to the conventional PNC materials.By using all-atom MD simulations,we find that the attractive interfacial hydrogen bonding interaction can induce the formation of dense P2VP layer,blocking the further interpenetration of P2VP chains into interior zone of OAPS.The thickness of interfacial layer is proved to be?1 nm,comparable to the size of OAPS.The absorption/desorption event analysis demonstrates that the diffusion of OAPS is governed by the“vehicle”-model process,that the OAPS NP are absorbed on surrounding polymer segments and using them as“vehicles”within a characteristic desorption time.Such findings provide direct microscopic insights for experiments and theories.The interfacial zone within OAPS/P2VP system shows“solid-liquid”form.The SCNP(single-chain crosslinked nanoparticle)has similar size as OAPS.However,SCNP is fabricated by intramolecularly crosslinking and keeps the soft nature of linear precursors.In addition,different from the attractive interactions at the OAPS/P2VP interface,SCNP has the same chemical composition as the linear matrix chain.By performing all-atom MD simulations,we investigated the interfacial interaction mechanism between PS SCNP and linear PS chains.The structural analysis shows that a thin layer with lower density formed within the interfacial zone of SCNP,which is structurally sparse with larger local free volume,leading to enhanced rotational relaxation of benzene rings and segmental dynamics on linear matrix chain.The above results provide direct evidence for the reduction of glass transition temperature of some PNC material with small NP.Besides,based on the above study,we developed a versatile reverse mapping method to insert atomistic details into coarse-grained model for the simulations of dense polymer melt with long chain length.SCNP melt as a special type of PNC,is intrinsically similar to linear polymers and nanoparticles.By using coarse-grained molecular dynamics simulations,we investigated the structure and dynamics of supercooled SCNP melts having a range of cross-linking degrees?.We find a nearly linear increase in glass transition temperature(T_g)with increasing?,and the increasing trends are consistent with recent experimental results.We attribute the increase in T_g to the enlarged effective hard-core volume and the corresponding reduction in the free volume of the polymer segments.Topological constraints evidently frustrate the local packing.In addition,the introduction of intra-molecular cross-linking bonds slows down the structural relaxation and simultaneously enhances the local coupling motion on the length scales within SCNPs.Consequently,a more pronounced dynamical heterogeneity(DH)is observed for larger?,as quantified by measuring the dynamical correlation length through the four-point susceptibility parameter,.The increase in DH is directly related to the enhanced local cooperative motion derived from intra-molecular cross-linking bonds and structural heterogeneity derived from the cross-linking process.